Re: latest pahole breaks libbpf CI and let's talk about staging

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Jun 17, 2021 at 11:48:47AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Wed, Jun 16, 2021 at 03:36:54PM -0700, Andrii Nakryiko escreveu:
> > On Wed, Jun 16, 2021 at 1:41 PM Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> > > And if I use pahole's BTF loader I find the info about that function:

> > > [acme@seventh linux]$ strace -e openat -o /tmp/bla pfunct -F btf tcp_cong_avoid_ai  ; grep vmlinux /tmp/bla
> > > void tcp_cong_avoid_ai(struct tcp_sock * tp, u32 w, u32 acked);
> > > openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 3

> > > So this should be unrelated to the breakage you noticed in the CI.

> > > I'm trying to to reproduce the CI breakage by building the kernel and
> > > running selftests after a reboot.

> > > I suspect I'm missing something, can you see what it is?

> > Oh, I didn't realize initially what it is. This is not kernel-related,
> > you are right. You just need newer Clang. Can you please use nightly
> > version or build from sources? Basically, your Clang is too old and it
> > doesn't generate BTF information for extern functions in BPF code.

> Oh well, I thought that that clang was new enough, the system being
> Fedora rawhide:

> [acme@seventh ~]$ clang -v |& head -1
> clang version 12.0.0 (https://github.com/llvm/llvm-project 87369c626114ae17f4c637635c119e6de0856a9a)

> I'm now building the single-repo main...

So I updated clang and now I'm stumbling on another one, again using
pahole 1.21 + fixes, without any of my changes, is this a known issue?

[root@seventh bpf]# pwd
/mnt/linux/tools/testing/selftests/bpf
[root@seventh bpf]# git log --oneline -5
94f0b2d4a1d0 (HEAD -> master, torvalds/master) proc: only require mm_struct for writing
a33d62662d27 afs: Fix an IS_ERR() vs NULL check
009c9aa5be65 (tag: v5.13-rc6) Linux 5.13-rc6
e4e453434a19 Merge tag 'perf-tools-fixes-for-v5.13-2021-06-13' of git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux
960f0716d80f Merge tag 'nfs-for-5.13-3' of git://git.linux-nfs.org/projects/trondmy/linux-nfs
[root@seventh bpf]#
[root@seventh bpf]# make run_tests
<SNIP>
  GEN-SKEL [test_progs] atomic_bounds.skel.h
  GEN-SKEL [test_progs] atomics.skel.h
  GEN-SKEL [test_progs] bind4_prog.skel.h
libbpf: elf: skipping unrecognized data section(6) .rodata.str1.1
  GEN-SKEL [test_progs] bind6_prog.skel.h
libbpf: elf: skipping unrecognized data section(6) .rodata.str1.1
  GEN-SKEL [test_progs] bind_perm.skel.h
  GEN-SKEL [test_progs] bpf_cubic.skel.h
libbpf: ELF relo #0 in section #15 has unexpected type 2 in /mnt/linux/tools/testing/selftests/bpf/bpf_cubic.o
Error: failed to link '/mnt/linux/tools/testing/selftests/bpf/bpf_cubic.o': Unknown error -22 (-22)
make: *** [Makefile:456: /mnt/linux/tools/testing/selftests/bpf/bpf_cubic.skel.h] Error 234
[root@seventh bpf]# clang -v |& head -2
clang version 13.0.0 (https://github.com/llvm/llvm-project dee2c76b4c46e71903e3d86ab7555a80d51d1288)
Target: x86_64-unknown-linux-gnu
[root@seventh bpf]#

- Arnaldo
 
> Would you consider a patch for libbpf that would turn this:
> 
> > > > libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2
> > > > Error: failed to open BPF object file: No such file or directory
> > > > make: *** [Makefile:460: /mnt/linux/tools/testing/selftests/bpf/bpf_cubic.skel.h] Error 255
> > > > make: *** Deleting file '/mnt/linux/tools/testing/selftests/bpf/bpf_cubic.skel.h'
> > > > make: Leaving directory '/mnt/linux/tools/testing/selftests/bpf'
> 
> Into:
> 
> libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2
> HINT: Please update your clang/llvm toolchain to at least cset abcdef123456
> HINT: That is where clang started generating BTF information for extern functions in BPF code.
> 
> ?
> 
> :-)